Rotation lifting units for operating the heddle frames of a weaving machine have become known on the one hand from British Pat. No. 1 089 821 and German Pat. No. 1 535 258, in which the coupling between the drive shaft and eccentric ring is effected by two individually movable, side-by-side keys which can engage the same groove, the keys having a common control member. The main characteristic is that each key engages the groove under the action of an initially tensioned spring. The advantage of these embodiments is the capability for anticipatory control, namely, initiating the control which will cause one key to move into the groove before the groove is positioned fully in front of the keys.
On the other hand, U.S. Pat. Nos. 3,726,323 and 3,724,511 disclose rotation lifting units for operating the heddle frames of a weaving machine in which a single key effects the coupling under control of a swingable switching arm with an indexing ring in its center, the key being controlled during a standstill of the drive shaft. In practice, this key has been provided with springs, by which it is held during the rotation of the drive shaft.
In the case of the first mentioned lifting units, there exists the disadvantage that the springs, in order to assure a quick movement of the keys, must be strong and dimensioned correspondingly large. Their physical dimensioning, however, is limited by the fact that they must be arranged within the thickness of one lifting unit, for example the usual 12 mm.
One purpose of the invention is thus the provision of a key coupling which permits a high speed of the drive shaft to be achieved and which eliminates the mentioned disadvantages, in particular installation of springs for biasing the keys.